@ujav Yes, wrong forum, but........
Are you measuring 3V relative to 0V.....?
The delay is the time during which it will stay in its "on" i.e triggered state, regardless.
Think "automatic garden lighting"...... which is probably what it was designed for.
If it was designed for security I would not expect a delay setting, but I don't know.
Then, probably, it will refuse to react for about 3 seconds if the "one time only" switch is set, because otherwise it could re-trigger itself if it were controlling a "hot" light.
If that is not set then it will trigger again straight away.
Which suggests that your 3V is actually "off", and that it is actually reacting to IR changes or noise all the time........ being continuously triggered by your presence.
Have you tried playing with the sensitivity (if it is like this one)...... http://qqtrading.com.my/pir-motion-sensor-module-hc-sr501
And also, thinking "automatic outside lighting" again, is it not triggering at all because there is too much ambient light? (I don't think that is the case..... I would still guess that it is being permanently triggered).